Gain control circuit



Oct 1.5, 1935., J. A. coY El AL 2,017,654

GAIN CONTROL CIRCUIT Filed Oct. 17, 1931 I 5 Sheets-Sheet 1 J. ,4 .cor 105 NVENTORSHLROMNES 0d. 15, 1935 J, co r AL 2,017654 GAIN CONTROL CIRCUIT Filed Oct. 17, 1951 3 Sheets-Sheet 2 J.A.C0V VENT-ORSHI. ROMNES ATTORNEY Patented Oct. 15, 1935 2,017,654

UNHTED STATES PATENT OFFICE GAIN CONTROL CIRCUIT Jay A. Coy, Glen Ridge, and Haakon I. Romnes, Bloomfield, N. J., assignors to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application October 17, 1931, Serial No. 569,360

16 Claims. (Cl. 178-44) This invention relates to control circuits for lines n acco d W th the Changes Caused y adjusting the gain on signal transmission lines temperature variation. Apilot line which is assoand particularly to gain contreI ir it t ciated with the toll lines and which is subjected peater stations on telephone t ll lines, to the same temperature variations as the toll One object of the invention is to provide a gain lines is emp y t on rol the ain at h r 5 control system for controlling the gain at a repeater station. In the case of cable toll circuits, peater station on toll lines that shall compensate one p f Wir s whi h is subj cted to av r e for changes in attenuation on the toll lines due to temperatur Conditions s selected as the Pilot temperature changes in a new and improved manline.

ner. The pilot line which is associated with the toll 10 Another object of the invention is to provide a lines and which is subjected to the same temperagain control system for a repeater station on a ture variations as the toll lines forms one arm Of signal transmission line that shall vary the gain a imp dance Wheats b e e ell A on the line at the repeater station under the conv n m r whi h i nne across pp i l5 trol of a pilot line to compensate for attenuation vertices of the Wheatstune bridge Circuit is Operchanges on the line in a manner t r e t bin accordance with the variations in attenujectionable clicks on the line and to insure against t n O t e Dilet linee gelvehemeter p intcrference with the transmission of signals on ates a mechanism of the yp disclosed in the C. the line in case of failure of power on the gain A- Borland et a1. Patent No. 1,717,8 dated June control system. 18, 1929, for Controlling a main motor and a gain 20 Another object of the invention is to provide control motor. The main motor op es a s de a gain control system for a repeater station on a Wire resistance element for restoring the balance signal transmission line that shall govern a galof the W o n brid e circui a r h p rvanometer according to the attenuation variaation of the galvehemeter- The main meter 1- tions on a pilot line associated with the transo op a centering m l m for insuring 25 mission line and control a reversible motor 2.0- the Operat on of the main meter e fixed p cording to the operation of the galvanometer to a number of fixed steps for eac Operation the control the number of impedance pads in the galvanometer. transmission line at the repeater station in an The gain control motor which is operated in a improved manner. forward and a reverse direction in accordance 30 A further object of the invention is to prowith the operation of the galvanometer serves vide a gain control system having a pilot line asto govern the Operation of a plurality of Cam sociated with asignal transmission line and formmembers which in turn con rol a number of ating one arm of a Wheatstone bridge circuit, a galtenu'ation or impedance pads in the toll line or vanometer connected to the bridge circuit and lines at a repeater station. The gain control cam 35 operated according to the attenuation variae ements direct y Operate sWitehihg elements tions on the pilot line, a main motor controlled which in turn control the insertion and exclusion by the galvanometer to restore the bridge balance of the pads f m h toll l A centering m and a gain control motor governed by the galis also associated with the gain control motor for 40 vanometer that shall operate a plurality of cam insuring that the gain nt l motor operates a 40 members according to the operation of the gain fixed step or a number of fixed steps for each control motor to govern a number of impedance ep a 0f the galvehemeter- The term pads at repeater station on the signal transfixed step when used to describe each movemission line. ment of the main motor or the gain control mo- The attenuation of a signal transmission line tors is intended to mean a step of unit amount so 45 "*iss according to temperature changes. It is that all steps in the operation of the motors are cssary to compensate for such changes of atequal. to nation by reason of temperature changes to A dial switch which is operated by the gain obtain transmission characteristics. It is control motor cooperates with switching mechacustomary to vary the attenuation of a line by nism operated by the main motor for operating 50 van the gain at repeater stations. an alarm in case the gain control motor does not In a gain control system constructed in acoperate in accordance with the operation of the cordance with our invention, the gain at a regalvanometer and the main motor.

ster station for one or any number of toll lines In the disclosed gain control system eight gain s varied to correct the a uati n o th e r control cam elements are associated with each 56 repeater for controlling pads on opposite sides of a toll line. The gain control motor preferably will operate five sets of gain control cams on five different toll lines. Only one main motor and one galvanometer are required for controlling the gain on all the pairs of toll lines in one toll cable. In a cable having, for example, 300 pairs of toll lines, a set of eight gain control cams and a set of eight pads will be required for each toll line. One gain control motor will be required as before set forth, for operating five sets of gain control cams and five sets of gain control pads; thus, 60 gain control motors will be required for controlling the gain at the repeaters on the 300 pairs of toll lines. The 60 gain control motors will be associated with one galvanometer and one main control motor.

The gain control cams which are formed in units of 8 each have 41 different positions to effect 41 different combinations of the impedance pads in a toll line. The gain control cams are operated in a forward and a reverse direction according to the direction of operation of the gain control motor to increase or decrease the gain in a toll line at the repeater station. Very little power is required to operate the main motor and the gain control motors and in case of failure of the power supplied to these motors, no objectionable interference with the transmission of signals over the toll line would be effected. It has been customary in some cases to control pads at the repeater station by relays. Objectionable clicks are caused by the operation and release of relays in the normal operation of the circuits. If the power fails, all relays release causing the repeater gain to be so widely changed that the circuits become commercially useless.

In the accompanying drawings, Figs. 1 and 2 taken together disclose a galvanometer controlling a gain control system constructed in accordance with the invention.

Fig. 3 is a diagrammatic view showing the connection of the pilot line to the Wheatstone bridge circuit for operating the galvanometer.

Fig. 4 is a developed view of the gain control cams.

Referring to Fig. 3 of the drawings, a pilot line I is assumed to be one pair of lines in a cable and to be subjected to the same conditions, as for example, temperature, as the other pairs of toll lines in the cable. The pilot line 1 forms one arm of a Wheatstone bridge circuit 2. The other arms of the Wheatstone bridge circuit 2 are preferably resistance elements 3. A galvanometer 4 is connected across two opposite vertices of the Wheatstone bridge circuit and a suitable source of current 5 is connected across the other two vertices of the bridge circuit. A slide Wire resistance element 6 which is controlled by the galvanometer 4 in a manner to be described hereinafter serves to restore the balance of the bridge circuit after each operation of the galvanometer 4.

The gal anometer 4 is operated in accordance with the variations in resistance of the pilot line I and accordingly is operated according to the variation in attenuation of the toll lines in the cable carrying the pilot line. In Figs. 1 and 2, the galvanometer 4 is shown controlling a galvanometer mechanism 1 for governing the operation of a main motor 8 and a plurality of gain control motors 9. Only one gain control motor 9 is shown on the drawings. The other gain control motors are similar in construction and operation to the illustrated motor 9 and operate apparatus similar to the apparatus shown operated by the illustrated motor 9.

The galvanometer mechanism 1 controlled by the galvanometer 4 comprises a U-shaped rocker arm It which is pivotally mounted at points ii. The pointer member E2 of the galvanometer 4 rides on a U-shaped supporting member 13 which is attached to the rocker arm Ill. The member iii limits the movement of the galvanometer pointer I2. An arm i4 secured to the rocker arm Ill engages a cam member E5 on a constantly rotating shaft IS. The shaft i5 is preferably connected to a motor I"! by a suitable gearing it. The gearing i3 also connects the motor ll to a recording paper roll is. The constant rotation of the cam member I5 serves to raise the rocker arm ill at predetermined intervals. Preierably the rocker arm is operated approximately every eight seconds. A flexible arm 23 engages a second constantly rotating cam member 21 on the shaft IS. A cross arm 22 is pivotally mounted on the resilient arm 29 and carries a pair of shoes 23 which are adapted to frictionally engage a disc 24. The disc 24 by means of a flexible connecting member 25 serves to control the movement of a recording pen 26. The cross arm 22 terminates in a pair of projecting lugs 21 which are located in a position such that a pair of rotating earns 28 on the constantly rotating shaft IE3 will normally not engage them. A plate 539 is fixed to the cross arm 22 and car ries a pair of pins 36. Pivotally mounted arms iii are held in engagement with the pins 36 by any suitable spring means not shown. The arms 3! are provided with projections 32 positioned above the pointer Q2 of the galvanometer 4. When the galvanometer needle or pointer I2 is deflected in one direction or the other it will under one of the projections 32 attached to the arms 3! and upon the rising of the rocker arm Ill under control of the cam l5 on the constantly rotating shaft l6, one of the arms 3i will be given a movement of rotation to rotate the cross arm 22 on its pivotal support. The rotative movement of the cross arm 22 places one of the projections 21 in the path of movement of one of the cam members 28. The cam member 23 upon engaging the projection ll on the cross arm 22 moves the cross arm back to its original position. At this time the cam member 2| will act to allow the shoes 23 on the cross arm 22 to frictionally engage the disc 2-; and as the rocker arm is given a movement of rotation, it will give a like movement oi rotation to the disc 24 for controlling the recording needle 26.

The projections 32 on the arms 3i not only control the rotation of the disc 24 for recording purposes, but also control two switches 33 and 34. The switch 33 serves to place a ground on a conductor 35 when the'galvanometer 4 is operated to reduce the gain at a repeater station. The switch 34 places a ground on a conductor 36 when the galvanometer is operated to increase the gain at a repeater station.

The switches 33 and 34 controlled by the galvanometer 4 serve to operate the main motor 8 in a forward and in a reverse direction and also to corn trol all of the gain control motors ll. The main motor 8 is preferably a shunt wound motor having the field winding thereof directly connected to suitable power conductors 31. The armature of the main motor 8 is connected to the power conductors 3'! by means of relays 3t and 39, the relay 38 serving to operate the motor 8 in one direction and the relay 39 serving to operate the motor in a reverse direction. The main motor 8 is geared to a shaft carrying a centering cam member 49 and the slide Wire resistance 6. The slide Wire resistance serves to restore the balance of the Wheatstone bridge circuit upon operation of the galvanometer 4. Two brushes 4| are provided for completing the circuit through the slide wire resistance element 5.

The centering cam 45 is provided with cam projections 42 equal in number to the steps or changes in the gain that can be effected at the repeater station. In the system under consideration, the cam member is assumed to have 41 cam projections corresponding to the 41 different steps of gain that can be eiiected at the repeater station. The centering cam 45 operates a switch member 43 which engages a contact member 44 or a contact member 45. When the switch member 43 engages the contact member 44, a ground is placed upon a conductor 46 for operating an alarm in case any one gain control motor does not follow the operation of the main motor 8' and the galvanometer 4. When the switch member 43 engages the contact member 45 a relay 41 is operated.

Assuming the switch 33 is closed by the galvanometer 4 for reducing the gain at the repeater station, a circuit is completed from a grounded battery 59 through the coil of a relay 5| and a switch arm of a relay 52. The relay 5! operates a switch arm 52A for placing a ground on a conductor 53, a switch arm 54 for establishing a locking circuit for the relay through a switch arm controlled by the relay 41, and a switch arm 55 for completing a circuit to ope-rate the relay 38. The relay 5| also operates a fourth switch arm 55 for effecting operation of the relay 52 to break the operating circuit for the relay 5|. Upon operation of the relay 38, a circuit is completed from the power conductors 31 through the armature winding of the main motor 8 to efiect operation of the main motor in a direction for operating the slide wire resistance to restore the balance of the Wheatstone bridge circuit. The centering cam 40 is also operated by the motor 8. When the centering cam is moved to the position whereby the switch member 43 engages the contact member 45, a circuit is completed from the grounded battery 58 for operating the relay 41. The relay 41 operates a switch arm 59 for breaking the holding or locking circuit for the relay 5| and for establishing a locking circuit for the relay 38.

The switch arm of relay 52 is released upon release of relay 5| by the operation of the relay 41 under control of the centering cam 49. As soon as the centering cam 49 has moved one step to the next cam projection, the switch member 43 disengages contact member 45 and engages the contact member 44. A ground is placed upon the conductor 45 and the relay 41 is released to release the relay 38 and stop the operation of the main control motor 8.

In case the galvanometer 4 is operated to increase the gain at the repeater station, the switch 34 is closed to place a ground on the conductor 36. The placing of a ground on the conductor 35 operates a relay 653 from a grounded battery in the same manner as the relay 5| is operated from the grounded battery 50 when ground is placed upon the conductor 35 by the switch 33. Relay 59 is similar in construction and operation to the relay 5! and upon operation serves first to establish a locking circuit for itself through a contact arm of the relay 41; second, to place a ground upon the conductor 6| third, to effect operation of the motor relay 39; and fourth, to effect operation of a relay 62 which corresponds to the relay 52 associated with conductor 35. The relay 62 breaks the energizing circuit for the relay 69 and leaves relay 60 under the control of the 5 relay 41 governed by the centering cam 46. The operation of the motor relay 39 completes a circuit through the armature winding of the main motor 8 from the power conductor 31 for effecting ope-ration of the motor in a reverse direction. Theoperationof the motor 8 serves again to govern the slide wire resistance 6 to restore the balance of the Wheatstone bridge circuit and to operate the centering cam 40. Centering cam 4|] first operates the relay 41 for releasing the relays 69 15 and 62 and then breaks the operating circuit of the relay 41 to release the motor relay 39 and stop the operation of the motor. The centering cam 40 limits the operation of the main motor 8 to a fixed step. If the wire resistance 6 is not ad- 20 justed sufiiciently by one motor step to balance the bridge circuit then the main motor is operated step by step until the bridge circuit is balanced. The conductors 53, 6| and 46 which extend from the relay mechanism controlling the 35 main motor 8 serve to control all the gain control motors 9 which govern the gain on all the pairs of toll conductors in the toll cable in which the pilot line is located.

In Fig. 2 of the drawings, conductors 53, El and 30 45 are shown connected to mechanism for operating one gain control motor 9 shown in Fig. 1 of the drawings. Conductors 62 which are connected to the conductors 53, 6| and 46 are assumed to control the operation of another gain 35 control motor.

The gain control motor 9 shown in Fig. 1 of the drawings is preferably a shunt wound motor having the field winding directly connected to a source of power comprising conductors 55. The 40 armature winding of the gain control motor 9 is connected to the power conductors 55 by means of motor relays 66 and 61, the relay 65 when operated serving to effect rotation of the motor 9 in one direction and the relay 51 when operated 45 serves to effect rotation of the motor 9 in a reverse direction. The relays 55 and 31 operate in controlling the gain control motor 9 similar to the operation of the relays 33 and 39 which control. the operation of the main motor 8. 50

The gain control motor 9 is geared to a shaft 58 carrying a centering cam 69 similar to the centering cam 49 which is operated by the main motor 8. a dial switch 15 and a set of gain control cam members A, B, C, D, E, F; G. and H. 55 Although only one set of eight gain control cam members is shown mounted on shaft it is to be understood that a number of similar gain control cam members are operated by each gain control motor. Preferably each gain control motor 60 operates five sets of gain control cam members.

One set of gain control cam members is shown developed in Fig. 4 of the drawings. Each cam of the set controls a switch member which in turn controls a pad in the east to west channel at the 55 repeater station and a pad in the west to east channel at the repeater station. In Fig. 1 of the drawings the cam member A is shown controlling a gain control switch 1| which in turn controls an impedance pad 12 in the east to west channel of the repeater and an impedance pad 13 in west to east channel of the repeater. The cam member H is shown controlling a switch member 14 similar to the switch member 1|. Switch member 14 controls two pads 15 and 16. The

cam members B to G inclusive between the cam members A and H operate switches similar to switches H and M which in turn control impedance pads similar to the impedance pads I2 and I3 controlled by the cam member A and impedance pads I5 and I6 controlled by the cam member H. Preferably the pads controlled by the various cam members are not of equal value in order to obtain a greater range of gain control. The impedance pads controlled by the cam member A may compensate for /2 db, the pads controlled by the cam member 13 may compensate for 1 db, the pads controlled by cam members C and D may compensate for 2 dbs, and the pads controlled by the cam. members E, F, G and H may compensate for 4 dbs.

The pad I2 is shown comprising an impedance element 95 which is adapted to be connected in series with the east to west side of the repeater and an impedance element 86 which is adapted to be connected across the east to west side of the repeater. The switch II which is operated by the cam member A is shown engaging a cam projection on the can member A and when in such position the impedance element is in series with the east to west side of the repeater and the impedance element 85 is connected across the east to west side of the repeater. The switch II at the same time effects a like connection of impedance elements comprising the pad '53 in the west to east side of the repeater. When the cam member A is moved so that the switch member TI does not engage the cam projection, then the impedance element 85 is shunted and the shunt connection of the impedance element 88 across the line is opened. The switches controlled by the cam members B to H inclusive are imilar in construction and operation to the switch member II and the pads associated with the other switches are similar in construction to pads I2 and '13 except that the pads vary in size. An amplifier i7 is shown in the west to east side of the repeater included in Fig. 1 of the drawings and an amplifier 18 is shown in the east to west side of the repeater. Preferably the amplifiers I1 and I8 are of the thermionic type.

Assuming the galvanometer 4 is operated to close the switch 33 and reduce the gain at the repeater station, the mechanism associated with the main motor 8 is operated as before set forth and at the same time a ground is placed on the conductor 53 by the switch arm 52A operated by the relay 5I The ground placed on the conductor 53 completes acircui't froma grounded battery for operating a relay I9. The relay I9 operates a switch arm 89 for completing a locking circuit for itself through a switch arm controlled by a relay BI. A second switch arm 82 operated by the relay I9 completes a circuit for operating the motor relay 68. The motor relay 66 completes a circuit from the power conductors 65 for operating the motor in a direction to increase the number of resistance pads in the gain control circuits at the repeater station.

Upon operation of the gain control motor 9, a switch 61 is closed by the centering cam 69. The closure of switch 81 completes a circuit for operating relay BI. Relay 8| operates a switch member 88 for establishing a locking circuit through the relay 6'5 and at the same time opens the holding circuit for the relay I9. The switch 81 is held closed until the motor 9 has made a complete step. The relay 8| is then released to open the locking circuit for the motor relay 66. The relay 66 opens the circuit of the motor 9 for preventing further operation of gain control cams. The gain control motor 9 is limited in its operation to a fixed step by the centering cam 69 in the same manner as the main motor 8 is limited to a fixed step by the centering cam 40. In such manner the cam members A to H inclusive are operated fixed steps to vary the pads in the line at the repeater station fixed amounts.

If the galvanometer 4 is operated in a direction to close the switch 34 for increasing the gain at the repeater station, the relay 66 as hereinbefore set forth is operated for effecting operation of the main motor 8. At the same time relay 69 grounds the conductor 6! which controls the operation of the gain control motor 9 in a predetermined direction. The grounding of conductor 6| completes a circuit for operating a relay 90. The relay 90 operates the switch member 9| for completing a circuit for operating the motor relay 61. A second switch member 92 is operated by the relay 90 for establishing a looking or holding circuit for the relay 90 under control of relay 8 I. The motor relay El upon operation completes a circuit from the power conductors t5 through the armature of the motor 9 for operating the motor in a reverse direction to that which it had been operated by means of the re lay 55. The cam members A to H inclusive will be operated in a reverse direction for controlling the switches operated thereby to reduce the number of pads included in the line at the repeater station. The centering cam 69 again closes the switch 87 for operating the relay 8|.

completes a holding circuit for the motor relay 6?. When the gain control motor is operated a fixed step, the centering cam 69 opens switch Bl for releasing the relay 8|. The relay 8I releases the motor relay 6! which in turn opens the circuit of the gain control motor 9.

The dial switch 10 which is operated by the gain control motor 9 serves to operate an alarm in case the gain control motor 9 and the cam member operated thereby do not follow the operation of the main motor 8 and the galvanometcr 4. In Fig. 1 of the drawings, the alarm is shown in the form of a lamp 93. The dial switch I0 comprises a grounded switch arm 94 which is mounted to rotate with the shaft 68. The switch arm 94 engages contact members 95 which are alternately connected to conductors 96 and 91. The number of contact members 95 is equal to the number of gain control steps that can be effected in the system. In the system under consideration, the number of steps is assumed to be 41.

Assuming that the switch arm 94 of the dial switch is in engagement with a contact member 95 connected to the conductor 91 when ground is placed on the conductor 53 by operation of the relay 5| when the galvanometer t closes the switch 33, a relay 99 is operated by the grounding of the conductor 53. he relay 98 operates a switch member 99 for completing a circuit to effect operation of a relay I09. The circuit for operating the relay I00 may be traced from grounded battery IUI through the coil of relay IIIU, switch member I92 of relay I93, switch member I94 of relay I05, switch member 99 of relay 98 and ground return to the battery ME. The relay IfiiJ operates a switch member Hill for connecting a circuit from a grounded battery Ill! The relay F iii breaks the holding circuit for the relay 9!) and through the lamp 93 to a contact member I08 75 adapted to be engaged by a switch member I09 operated by the relay 98. At this time the relay 98 is assumed to be operated by the ground on the conductor 53 which is established by the switch member 52A of the relay 5i. However, when the main motor 8 has completed its operation and the relay 5| is re eased to release relay 98, then the switch member I09 is released to engage the contact member I98. At this time if the gain control motor 9 fails to follow the operation of the main motor 8, a circuit will be completed from the battery I91 through the lamp 93, contact member I 06, contact member I08, switch member I09, and a ground return established by the switch member 43 controlled by the centering cam 40 of the main motor. If such circuit is completed, the lamp 93 will be lighted. However, such circuit will only be completed in case the gain control motor 9 fails to follow the operation of the main motor.- In the case under consideration, it has been assumed that the switch arm 94 has not moved away from the contact member 95 connected to the conductor 91. If the switch arm 94 had moved one step so as to engage a contact member 95 connected to the conductor 96, then the circuit for operating relay I03 would have been opened to prevent operation of the alarm.

If the switch arm 94 of the dialing switch is in engagement with a contact member 95 connected to the conductor 96, then a relay III! will be operated in place of the relay I93 when the gain control motor 9 fails to follow the operation of the main motor. The relay 98 is operated by ground on the conductor 6| in the same manner as it is operated by a ground on the conductor 53. When the relay I I0 is operated by the dialing switch, a relay I05 is operated in the same manner as relay I 00 for establishing a circuit through the lamp 93. The relays I93 and III] prevent operation of the alarm except when the gain control motor fails to complete its operation.

The developed view of the cam members A to H inclusive shown in Fig. 4 of the drawings indicates the 41 steps of the gain control system. The squares which are in cross-section on the various cam members indicate cam projections for operating switches to insert the pads in the line. For example, in position I no pads are inserted in the line; in position 2 the pad controlled by the cam member A is inserted in the line; in position 3 the pad controlled by the cam member B is inserted in the line; and in position 4 the pads controlled by the cam members A and B are inserted in the line.

It is to be understood that although only one set of cam members is shown connected for operation by the gain control motor 9, that at least four other sets of cam members are operated by the gain control motor. Furthermore, it is to be understood that any desired number of gain control motors 9 may be governed and controlled according to the operation of the main motor 8 and the galvanometer 4.

Modifications in the system and in the arrangement and location of parts may be made within the spirit and scope of the invention and such modifications are intended to be covered by the appended claims.

What is claimed is:

1. In a gain control system for a transmission line, a Wheatstone bridge circuit having one arm thereof in the form of a pilot line associated with said transmission line, a galvanometer connected to said bridge circuit and operated according to the variations of a characteristic of said pilot line, means comprising a main motor of a continuously operable type controlled by said galvanometer for restoring the bridge circuit balance upon operation of the galvanometer, means operated by the main motor for limiting each movement of said motor to a fixed step so that the motor steps are equal, and means comprising a control motor governed by said galvanometer for governing the gain on the transmission line.

2. In a gain control system for a transmission line, a pilot line, a Wheatstone bridge circuit having three impedance arms and one arm formed by said pilot line, a galvanometer connected across opposite vertices of said bridge and operated according to a characteristic of said pilot line, means comprising a main motor of a continuously operable type controlled by said galvanometer for restoring the balance of the bridge upon operation of the galvanometer, means operated by said main motor for insuring operation of the main motor by equal steps when governed by the galvanometer to balance the bridge circuit, and means comprising a control motor governed by the galvanometer for governing the gain on said transmission line.

3. In a gain control system for a transmission line having a repeater station therein, a pilot line associated with said transmission line, a Wheatstone bridge circuit having three fixed impedance arms and a fourth arm comprising said pilot line, a galvanometer connected across two opposite vertices of said bridge circuit, means comprising a main motor controlled by said galvanometer for restoring the balance of said bridge circuit upon operation of the galvanometer, means operated by the main motor for effecting step-by-step operation of said motor by equal steps when balancing said bridge circuit, means comprising a second motor controlled directly by said galvanometer and independently of the operation of said main motor for adjusting the gain at the repeater station in said transmission line and means for effecting step by step operation of said second motor by equal steps when controlled by the galvanometer.

4. In a gain control system for a transmission line having a repeater station therein, a pilot line associated with said transmission line, a motor, a plurality of cam members operated by said motor, a plurality of impedance pads, switch members operated by said cam members for inserting said pads in the transmission line, means operated in accordance with a characteristic of said pilot line for governing the operation of said motor, a centering cam operated by said motor, and'means operated by the centering cam for limiting each operation of said motor to a fixed step.

5. In a gain control system for a transmission line having a repeater station therein, a pilot line associated with said transmission line, means comprising a gain control motor of a continuously operable type and a plurality of cam members operated thereby for controlling the gain on the transmission line at'said repeater station, control means operated in accordance with a characteristic of said pilot line for operating said motor and means operated by said motor for limiting the operations of said motor to fixed equal steps.

6. In a gain control system for a transmission line having a repeater station therein, a Wheatstone bridge circuit having one arm thereof in 75 the form of a pilot line associated with said transmission line, a galvanometer connected to opposite vertices of said bridge circuit and operated according to the variations in a characteristic of said pilot line, a control motor of a continuously operable type operated in a forward and in a reverse direction according to the operation of said galvanometer, means comprising a plurality of gain control cams operated by said control motor for controlling the gain at the repeater station, and means governed by the control motor for limiting each rotative movement of said motor under control of the galvanometer so that each motor movement is a fixed step and all steps are equal.

'7. In a gain control system for a transmission line having a repeater station therein, 3. Wheatstone bridge circuit having an arm thereof in the form of a pilot line associated with said transmission line, a galvanometer connected to opposite vertices of said bridge circuit and operated according to the variations in a characteristic of said pilot line, means comprising a main motor of a continuously rotatable type operated in a forward and in a reverse direction by said galvanometer for restoring the bridge circuit balance upon operation of the galvanometer, means controlled by the main motor for effecting operation thereof in a step-by-step manner, each step of the main motor being fixed and all steps being equal, a control motor operated in a forward and in a reverse direction by said galvanometer, means for operating the control motor in a step-by-step manner, each control motor step being fixed and all steps being equal, and means comprising a plurality of gain control cams operated by said control motor for controlling the gain at the repeater station.

8. In a gain control system for a transmission line having a repeater station therein, a Wheatstone bridge circuit having one arm thereof in the form of a pilot line associated with said transmission line, a galvanometer connected to opposite vertices of said bridge circuit and operated according to the variations in a characteristic of said pilot line, a plurality of gain control cams, means comprising a motor of the continuously operable type operated in a forward and a reverse direction according to the operation of said galvanometer for operating said cams to control the gain at the repeater station on the transmission line and means operated by the motor for controlling said motor to insure operation of the gain control cams in a step-by-step manner, each step being fixed and all steps being equal.

9. In a gain control system for a transmission line, a Wheatstone bridge circuit having one arm thereof in the form of a pilot line associated with said transmission line, a galvanometer connected to said bridge circuit and operated in accordance with variations in a characteristic of said pilot line, main control means governed by said galvanometer for restoring the bridge circuit balance upon operation of the galvanometer, gain control means mechanically independent of said main control means and controlled by said galvanometer for governing the gain on the transmission line, and means jointly controlled by the gain control means and the main control means for operating an alarm in case the gain control means fails to operate in accordance with the operation of the main control means.

10. In a gain control system for a transmission line having a repeater station therein, a Wheatstone bridge circuit having one arm thereof in the form of a pilot line associated with said transmission line, a galvanometer connected to said bridge circuit and operated in accordance with variations in a characteristic of said pilot line, main control means governed by said galvanometer for restoring the bridge circuit balance upon operation of the galvanometer, gain control means operated according to the operation of said galvanometer for controlling the gain at the repeater station on the transmission line, a dial switch operated with said gain control means, and means jointly controlled by said main, control means and the dial switch for operating an alarm in case the gain control means fails to: operate in accordance with the operation of the main control means.

11. In a gain control system for a transmission line, a Wheatstone bridge circuit having one arm thereof in the form of a pilot line associated with said transmission line, a galvanometer connected to said bridge circuit and operated in accordance with variations in a characteristic of said pilot line, means comprising a main motor controlled by said galvanometer for restoring the bridge circuit balance upon operation of the galvanometer, means comprising a control motor operated according to the operation of said galvanometer for controlling the gain at a repeater station on said transmission line, a clial switch operated by said control motor, and means controlled by said main motor and the dial switch for operating an alarm in case the control motor fails to operate in accordance with the operation of the main motor.

12. In a gain control system for a transmission line, a pilot line associated with said transmission line, a Wheatstone bridge circuit having one arm thereof formed by said pilot line, a galvanometer connected across opposite vertices of said bridge circuit and operated according to variations in a characteristic of said pilot line, a main motor, means for operating said main motor in a forward and a reverse direction according to the operation of said galvanometer, means operated by said main motor for restoring the balance of the bridge circuit upon operation of the galvanometer, a centering cam operated by said motor, and means operated by said centering cam for limiting each operation of the motor to a fixed step.

13. In a gain control system for a transmission line having a repeater station therein, a pilot line associated with said transmission line, a Wheatstone bridge circuit having one arm thereof formed by said pilot line, a galvanometer connected across opposite vertices of said bridge circuit and operated according to variations in a characteristic of said pilot line, a control rotative member, control means governed by said galvanometer for operating the control member in a forward and a reverse direction, means operated by said rotative member for governing said control means to limit each operation thereof to a fixed step, and means comprising a plurality of gain control cams operated by said rotative member for controlling the gain on the transmission line at said repeater station.

14. In a gain control system for a transmission line having a repeater station therein, a pilot line associated with said transmission line, a Wheatstone bridge circuit having one arm thereof formed by said pilot line, a galvanometer connected across opposite vertices of said bridge circuit and operated according to variations in a characteristic of said pilot line, a control motor, 16

means governed by said galvanometer for operating the control motor in a forward and a reverse direction, a plurality of gain control cams and a centering cam operated by said control motor, a plurality of impedance pads at said repeater stations, means operated by said gain control cams for controlling the number of said pads in the transmission line at the repeater station to govern the gain at the repeater station,

and means operated by said centering cam for limiting each operation of the motor to a fixed step.

15. In a gain control system for a transmission line having a repeater station therein, a pilot line associated with said transmission line, a Wheatstone bridge circuit having one arm thereof formed by said pilot line, a galvanometer connected across opposite vertices of said limiting each operation of the main motor to a fixed step, a control motor, means operated by the galvanometer for operating the control motor in a forward and a reverse direction, a plurality of gain control cams and a centering cam operated by said control motor, means operated by said last mentioned centering cam for limiting each operation of the control motor to a fixed step, and means operated by said gain control cams for governing the gain at the repeater station and the transmission line.

16. In a gain control system for a transmission line, a galvanorneter, means for operating said galvanometer according to a characteristic of said line, a main motor of a continuously operable type controlled by said galvanometer, means operated by said main motor for returning the galvanometer to initial position, means operated by the main motor for insuring operation of the main motor by equal steps, a: gain control motor of a continuously operable type directly controlled by said galvanometer, and means operated by the gain control motor: for insuring the operation of the gain control motor by fixed equal steps.

JAY A. COY. HAAKON I. ROMNES. 

